UFC 3-250-03
15 May 2001
condition will affect the sand adhesion. Current laboratory procedures for sample preparation should be
improved so that samples can be more consistently molded. For some aggregates, LWT has shown to
permit excessive amounts of binder resulting in unacceptable mixtures. This is true particularly for
applications in high shear areas such as intersections. Performance data indicates that mixtures
produced with these aggregates using a lower binder content (than would have been permitted by LWT)
have performed well in extending the pavement service life. The specific gravity specification is very
subjective due to sampling procedure. The entire LWT specimen is weighed wet and dry to obtain
specific gravity. After compaction the same test is repeated. The problem is only 50 to 60 percent of the
specimen is compacted. Variations in the specific gravities of samples can also skew LWT results.
[2] Marshall design. The applicability of this HMA test for micro-surfacing is
questionable. The Marshall series uses large specimens of varying asphalt contents which are dried,
reheated to 135EC, and compacted to low void content. Micro-surfacing mixtures neither reach these
temperatures nor do they compact to low design voids. Field observation has noted air voids of 10 to
15 percent after 1 to 2 years of placement. There is a need to correlate the voids measured during the
design using the Marshall method with the actual field voids. One materials laboratory that has
developed a cold Marshall test procedure to estimate field voids is currently correlating the field voids
with the voids obtained by the modified HMA procedure. The HMA samples are prepared by compacting
in a mold. The question whether the microsurfacing samples should be compacted or screened into the
sample mold remains to be answered. Also, for reliable results, the sample has to be cured in a
uniformly distributed film throughout the thickness of the lift.
e. Surface preparation. The pavement surface should have all loose material removed prior to
application of the micro-surface. Any structurally deficient pavement areas must be repaired. Cracks
wider than 3 millimeters (1/8 inch) or with vegetation should be cleaned and sealed prior to application of
the micro-surface.
f. Application. A tack coat should be applied to the pavement surface prior to application of the
micro-surfacing. Immediately prior to application, the pavement surface should be wetted with a water
fogging. This fogging should leave the surface damp but with no free water. The minimum thickness of
application should at least exceed the maximum nominal size of the aggregate in the mixture (usually
1-1/4 to 1-1/2 times). This relates to minimum thicknesses of from 10 to 15 millimeters (3/8 to slightly
over inch). Where wheel ruts exceed 6 millimeters (1/4 inch) in depth, a separate rut filling layer
should be placed prior to the complete overlay. The emulsion is generally heated to within 27 to 49EC
(80 to 120EF) prior to mixing. Micro-surfacing applications are generally designed to be opened to
traffic within 1 hour after placement. Temperature and humidity are the controlling factors for curing
micro-surfacing placement. As the temperature increases and the humidity decreases, the cure time
decreases. Construction of a test section is very important for micro-surfacing due to changes in field
conditions from lab conditions. Micro-surfacing is a quick-set system. Therefore, changes or variations
in field conditions may require moisture, additive, or basic mix design changes to meet field conditions.
Where possible, placement should be accomplished utilizing "nurse trucks" to allow for continuous
placement. "Nurse trucks" are vehicles that are intended to carry aggregate, emulsion, and water to the
application vehicle, whereby, application of the micro-surfacing can be a continuous operation.
Whenever placement of the micro-surfacing is stopped, the spreader box must be lifted and cleaned and
the transverse joint squared. Paper strips or metal flashing can be used to improve transverse joints.
Longitudinal joints should be constructed with a 50 millimeter (2 inch) overlap to assure complete
coverage and to reduce rigid development. The use of an operator to control the rate of material flow
along with careful control of the placement vehicle's speed allows for accurate placement of the micro-
surfacing.
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